Perhaps something like this with upgraded antennae (or even a wifi base-station in your attic near the GH if better antennas won't work) would work for you?
Wifi thermometer temperature email alert graph webcam | eBay

I have started thinking more laterally.
My internet service connects to my home by radio through an aerial on the roof (not by land line) . That little aerial is in line of sight from both greenhouses, so I am exploring the possibility of transmitting to that point then perhaps to softwear on my PC.
Not exactly what I intended, but better than nothing............but it all depends on whether there is a unit out there which can communicate with my internet aerial.

Richard

Hmmm, perhaps this is a little too simple, but I got a decent little temp gauge that reads the temp in my green house as well as the humidity and the temp in my home as well. Home Depot, it's from Accurite. cost me about $75, there were some that were in the 25 range that didn't give a humidity read out. I don't think I would feel "safe" with my orchids in the GH without it. I can even check and see what the highest and lowest temp was over the last 24 hours. totally affordable and super easy.

I'm guessing you mean one of the wireless systems with a sensor in the GH and a readout in your house that also shows the house conditions. Those are common and cheap but they have a very limited range. If your GH is connected or close to the house they work, but put it 100 ft or so away and you can't get a signal.

Once again I point out that remote reading systems are fine as long as you are looking at them. They are useless when you are asleep or otherwise not looking. That's why, in my case, I want something that notifies me that something is wrong. Thus my preference for an alarm like I have. As I mentioned before, my system is not limited to being an alarm. I can call it up anytime and get the current conditions.

Different things suit different situations. I lost my heat twice at night during very wintry conditions due to a malfunctioning relay in the controller. If I had not had an alarm, I would have lost all my plants. As it was I just had to sit up all night manually turning the furnace on and off. Could have been a disaster. Just became a PiTA.

Once upon a time.... I used a wired device. Sensor in the GH, display in my office, connected by 125' of buried wire. Inexpensive and reliable.

Until ice sliding of the greenhouse during an ice storm pierced the ground next to the foundation (where it stays thawed), severing the wire right where it left the GH, going into its buried channel.

Of course, all this happened coincident with a heater failure on a 7°F night, wiping out a 20 year collection of plants.

That is why control systems typically use 4-20 ma signals. If the signal drops below 4 ma then the system can recognize that there has been a signal failure and take appropriate actions such as issuing alarms.

You'll probably find your wireless internet is on a 5GHz link, and probably on a fairly directional antenna; I suspect unless you have access to the backend of that CPE, you're not going to win :(

I have been chasing this one for many months and must now admit defeat. There are systems out there but they are intended for commercial use and for an amateur the cost is telephone number size. Exploration with manufacturers cannot adapt the units nor reduce the price. I have lasted 15 years without such a facility, but I just thought that being retired and around the house most of the time I could just glance at something without tramping to my greenhouses. I know it is all proportional and with around 600 plants I need to be vigilant, but there comes a time when cost becomes overwhelming.
An interesting discussion topic though - thanks everyone.
Richard

DIY temperature monitor with failsafe alarm.
 

Here is a temperature sensor that provides a 4-20ma output ($125):
TTD25N-20-0100C-H Products

Here are panel display meters that accept 4-20ma input and have relay outputs for limit and loss of signal alarms ($280):
APM765 Panel Meter with Digital Display, Isolated 4-20mA and Modbus Output Options

Also a 4-20 ma with limit and loss of sensor alarm outputs but without display ($310):
260A limit alarm with DC current input and loop-power

Using 4-20ma signaling you can send the signal over a 2 conductor line for a thousand feet or more.

Check out Sero systems growpak 1... It a controller but you can use it to monitor your growing conditions... You can setup 4 modules. You can setup two RH/temps sensors and a net module two send live data to your computer or cellphone......


FYI: Herpkeeper is the same thing but cheaper.